Astronomers have detected the brightest explosion ever witnessed in the heavens. It was a flash of light from across our galaxy so powerful that it bounced off the Moon and illuminated Earth's upper atmosphere. The researchers say if the blast had occurred closer to Earth, it might have lead to the death of much of the planet's life.

The monstrous cosmic explosion took place late in December, observed by U.S. and European satellites and ground telescopes around the world. It was impossible to miss. It was brighter than a full moon for a fraction of a second, although it shone more brilliantly in high energy gamma rays than in the lower energy visible light.

Astrophysicist David Palmer of the U.S. government's Los Alamos National Laboratory says the amount of energy equaled 100,000 years worth of sunlight. "In other words, this was as much energy during that tenth of a second as all the other stars in the galaxy at that time, times over 1,000," he said.

The scientists who observed the flash say it came from an isolated neutron star in the constellation Sagittarius about 50,000 light years away, the distance it takes light to travel in 50,000 years. Astronomer Andrew Fruchter of the institute that operates the Hubble Space Telescope describes a neutron star as the spinning remnant of a massive star that has collapsed into a diameter of only 20 kilometers.

"Massive stars live fast, die young, and leave spinning corpses. What we've seen is that their afterlives are even more interesting than we had thought," he said.

The neutron star that caused the giant cosmic blast is an exotic type called a magnetar. It is so named because it has an extremely powerful magnetic field, 1,000 times stronger than an ordinary neutron star and a 1,000-trillion times that of Earth's. It is capable of erasing the magnetic information on a credit card at a distance halfway to the Moon. Only about a dozen magnetars are known among the billions of stars in the Milky Way.

Astronomers believe the star's magnetic field caused the outburst, but do not understand how. One possibility is that the blast was like an ultra-powerful solar flare. This is where a magnetic field changes shape and bumps into itself. Since magnetic fields cause electrical currents, such a collision could have caused a monumental short circuit visible from Earth.

Or a quake on the star's surface might have caused the massive eruption of energy.

Whatever the cause, the blitz of radiation expanded outward in a bubble at about one-fourth the speed of light. This energy soon showered Earth's upper atmosphere, tearing apart atoms in a region called the ionosphere to a greater depth than an ordinary solar flare and causing them to glow. It was so powerful that it pushed the instruments of 15 spacecraft off scale, whether or not they were pointing in its direction. It also saturated a U.S. telescope watching in x-ray wavelengths.

Researcher Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics says if the explosion had occurred much closer to our solar system, say just 10 light years away, it would have severely damaged the atmosphere and possibly triggered a mass extinction of living things.

"These objects do not normally get this big this quickly. We're used to planting an acorn in the ground and maybe waiting 50 years for it to turn slowly into an oak tree. This time, it was like we were standing in front of the acorn and it popped up into a full tree just in the space of a few weeks," he said. "This was so extreme that this may not happen again in our lifetimes."

The tremendous power of the event suggests a novel solution to a long-standing astronomical mystery. Scientists want to know the origins of the strange phenomenon of brief gamma ray bursts observed hundreds of times over recent decades. They say these short-duration flashes emit gamma rays in the same way the neutron star did in December and that such stars could explain a large number of the occurrences.

To get at this puzzle, the U.S. space agency launched a spacecraft named "Swift" last November with the most sensitive gamma ray detectors ever designed. David Palmer at the Los Alamos National Laboratory notes that, luckily, Swift saw this event after only one month on the job. "Sometimes being lucky is the best strategy," he said.